1. Field of the Invention
The invention is directed to wavelength division multiplexed (WDM) optical communication systems in general and, more particularly, to bidirectional WDM optical networks having optically-amplified bidirectional add-drop multiplexer nodes including an optical network control element for controlling optical launch powers.
2. Description of the Related Art
As the need for communication signal bandwidth increases, wavelength division multiplexing (WDM) has progressively gained popularity for multiplying the transmission capacity of a single optical fiber. A review of optical networks, including WDM networks, can be found in Ramaswami et al., Optical Networks: A Practical Perspective (Morgan Kaufman,.COPYRGT. 1998), the disclosure of which is incorporated herein by reference. Typically, wavelength division multiplexed optical communication systems have been designed and deployed in the long-haul, interexchange carrier realm. In these long-haul optical systems, a wavelength division multiplexed optical communication signal comprising plural optical channels at different wavelengths travels in a single direction on a single fiber (unidirectional transmission). Because the communication traffic in such systems commonly travels many hundreds of kilometers, the need for add-drop multiplexing of individual channels is infrequent (if at all), occurring at widely-spaced add-drop nodes.
Although the optical infrastructure of long-haul WDM optical systems can accommodate future traffic needs created by increased demand from traditional and multimedia Internet services, this traffic must first be collected and distributed by local networks. Currently, such local networks are structured to carry a single wavelength, time-division multiplexed (TDM) optical signal along a fiber network organized into various ring structures. To route the various components of the TDM signal, numerous electronic add-drop multiplexers are positioned along the fiber network. At each add-drop location, the entire optical signal is converted into an electrical signal; the portions of the electrical signal which are destined for that add-drop point are routed accordingly. The remaining portions of the electrical signal are converted back to a new TDM optical signal and are output through the electronic add-drop multiplexer. Thus, before a user can access the bandwidth-rich WDM long-haul transport networks, he must first pass through the bottleneck of the local networks.
Although unidirectional WDM optical systems are suitable for conventional long-haul interexchange carrier markets, metropolitan (local) communications systems require extensive routing and switching of traffic among various nodes positioned within interconnecting optical fiber rings. Consequently, smaller metropolitan markets require considerably more extensive add-drop multiplexing in order to successfully implement wavelength division multiplexing in their short-range systems. In conventional, point-to-point (e.g., long-haul) optical systems, the optical channels are launched from a single starting location and aggregated onto an optical fiber through the use of an optical combiner. Such systems are depicted in U.S. Pat. Nos. 5,504,609, 5,715,076, and 5,784,184. However, in metropolitan networks, particularly metropolitan ring networks, it is desirable to continuously add and drop optical channels simultaneously from multiple points around an optical ring. Thus, there is no "start" or "end" node in such a metropolitan network as in the prior art point-to-point networks.
Further, in order to maximize the effectiveness of wavelength division multiplexing in metropolitan networks, it would be useful to implement bidirectional WDM optical systems, e.g., to enhance network design flexibility and reduce the number of optical fibers needed to implement protection switching. In a bidirectional WDM system, counter-propagating WDM optical signals, each comprised of plural optical channels, are carried on the same waveguiding medium such as a single optical fiber. Implementation of a bidirectional system requires several considerations not present in conventional unidirectional optical systems. Add-drop multiplexing in a bidirectional optical environment becomes considerably more complex since optical channels must be selected from each of the counter-propagating WDM optical signals. Optical amplification also becomes more complex in a bidirectional optical network since amplification must be performed on each of the counter-propagating WDM signals. When the bidirectional system is deployed in a metropolitan network, the positioning of optical amplifiers is difficult due both to geographic (space) constraints for positioning optical nodes at irregular intervals and the need for the network to accommodate future expansion (e.g., future add-drop). When additional nodes are added to an existing WDM metropolitan network, the number and placement of optical amplifiers must be reconfigured to conform to the optical power budget for the new network.
Several bidirectional multiplexer designs have been proposed as well as several designs for bidirectional amplifiers; however, none of these permit the creation of easily reconfigurable or expandable bidirectional wavelength division multiplexed optical networks. In U.S. Pat. No. 5,909,295, optical circulators are used to separate the counter-propagating optical signals that are further filtered down to individual channel wavelengths; this design appears to be predominantly directed to an end node in a WDM optical system. In many embodiments, expensive four-port (or higher) optical circulators must be used. Although optical channels are separated, there is no teaching or suggestion of signal recombination such that a bidirectional optical signal continues to propagate along a bidirectional transmission waveguide.
In U.S. Pat. No. 6,130,765 a bidirectional add-drop multiplexer is described. The apparatus comprises two three-port main circulators inserted into a line fiber; the third port of each circulators connected by auxiliary optical fibers to auxiliary circulators. While this patent depicts channel routing in a single bidirectional line fiber, it does not teach or suggest an optically-amplified bidirectional optical network.
Bidirectional optical amplifiers arc also known in the art for use in conventional, point-to-point optical networks. Bidirectional optical amplifiers are shown in U.S. Pat. Nos. 5,742,416, 5,812,306, 6,101,016. In these patents optical signals are routed unidirectionally through a single optical amplifier or separated and routed through separate amplifiers. However, because these optical amplifiers are designed for use in point-to-point optical systems, they do not include any mechanism for adding and dropping optical traffic onto the optical network at the position of the bidirectional amplifiers.
Thus, there is a need in the art for a bidirectional WDM optical communication network capable of supporting add-drop multiplexing and optical amplification while permitting network expansion with future optical add-drop multiplexer nodes. Such systems would permit effective implementation of bidirectional wavelength division multiplexing in local, metropolitan markets requiring high volumes of signal re-routing and allow creation of flexible network topologies.